In semiconductor device manufacturing technologies, photolithography is typically used to transfer a pattern for forming semiconductor features onto a semiconductor wafer for the formation of integrated circuits. During a photolithographic process, light passes through a photomask to expose a photosensitive layer formed on a surface of the semiconductor wafer. The photomask includes predetermined circuitry patterns. The predetermined circuitry patterns may have attenuating regions and non-attenuating regions, so that the light can be modulated in both intensity and phase. In a typical photolithographic process, exposed portions of the photosensitive layer are developed to form a pattern for subsequent processes, such as etching features into underlying material layers.
As feature sizes of the semiconductor devices have decreased to sizes smaller than the wavelength of light used in photolithographic processes, diffraction or optical fringing and interference of light passing through a photomask become more significant in forming the features with small critical dimensions (CDs). The diffraction or optical fringing and interference of the light cause undesired light exposure on the photosensitive layer in undesired regions, thus resulting in loss of pattern resolution in transferring of the photomask pattern.
In order to increase the resolution of a transferred photolithographic pattern, a phase shift mask (PSM) has been developed. Phases of wavefronts of light passing through a photomask pattern of the phase shift mask are intentionally phase shifted in selected portions to selectively produce destructive interference, thereby reducing undesired light exposures of a photosensitive layer due to diffraction of light passing through the patterned photomask. However, the conventional PSM is not entirely satisfactory in all respects.